There's an interesting if ultimately rather academic discussion currently in progress on PoliticalBetting.com on who might win the Labour leadership if anything were to happen to Gordon Brown over the next few months - if he were to "fall under a No 13 bus" in the traditional Westminster parlance.
In the context of leadership speculation, the bus is used as a convenient shorthand for (i) a debilitating illness or family difficulty that might put a leadership candidate temporarily or permanently out of consideration, or (ii) the emergence of a sudden scandal that could engulf his or her hopes. Both are possibilities in Gordon's case, though unlikely.
In the event of GB being forced to pull out, Mike Smithson is backing David Miliband rather than John Reid to emerge as the frontrunner, although Miliband's September statement - "I am not a runner nor a rider for any of the jobs that are being speculated about" - appears to suggest he is not just ruling out a challenge to Gordon but making clear he will not be a candidate for the leadership in any circumstances.
For my part, I think both Miliband and Reid would struggle to build the kind of broadly based support within the party that would be needed to mount a successful challenge. Rightly or wrongly, they would both be seen as Blairite continuity candidates, and I think that, in the unusual circumstances of a Brown withdrawal, a compromise candidate would be certain to emerge.
The obvious name that springs to mind here is Jack Straw. He will not stand against Mr Brown, but I have always regarded it as a certainty that he would stand against anyone else. Jack knows his own worth, and as the next most experienced and senior figure in the Government after Mr Brown, he would regard himself as the natural successor were the Chancellor to be put hors de combat.
Margaret Beckett is also a possibility. She too would never oppose Brown, but she has an unerring habit of being in the right place at the right time politically, and it is more than conceivable that she could come through on the rails.
There is another factor which would put Straw and Beckett ahead of the likes of Miliband, and that is their age. If Gordon were put out of action, his people would look around for a caretaker leader who could be relied on to stand aside after four or five years, by which time Gordon might have overcome his personal or political difficulties.
Having observed Gordon Brown from a distance for a number of years, I am convinced he will never give up his ambitions of reaching No 10. And even if he were to be forced to do so temporarily, he will try to arrange things in such a way that he lives to fight another day.